Brian Haw calls it flying. 'We've been flying for three or four hours now, Tim,' he says to me on the hot pavement opposite the Houses of Parliament, where he has been protesting for more than four years about government policy in Iraq.

The aviation Haw is referring to describes the looping global polemic that makes up most of his conversation. He has touched down over the course of a long morning at various troubled points on the earth's surface, in Cambodia and Kashmir and, most frequently, Baghdad and Fallujah; I've been sometimes struggling to spot landmarks.

There was a time when Brian Haw used to go and see the world, but these days it comes to him. While we are sitting, flying, on the pavement, we are interrupted by camera crews from Norway and Zambia.

'It's a busy life here,' he tells them. 'One moment, I am speaking to Togo, the next to Australia. I'm told I was front-page news in the Lebanon. Mexican public radio gave me 45 minutes every day for a week.'

Everyone who comes wants to know the answer to the same question. The Zambian journalist blinks at Brian through thick glasses. 'Why are you here, sir?' he asks.

There is an obvious answer to this question, the one which Brian Haw repeats to anyone who will listen. It is the answer that is painted on the placards that surround him and that stretch along the railings. He is here for peace, to stop the war in Iraq. 'I am here for all the world's kids,' he tells the Zambian reporter. But there are, I guess, other more personal answers, too.

Brian keeps a little Biro reminder of the 1,500 or so days and nights he has spent on this pavement; Robinson Crusoe adrift on his traffic island. He sleeps under a green tarpaulin a few hours a night, sits on a deck chair most of the day. Facing Big Ben, he's never stuck for the time. He eats whatever he is given, soup or chips, washes in a bucket, take one shower a week at a friend's.

He has, so far, beaten five eviction orders. ('The last time the case was heard in front of a Judge Moses,' he says. 'I'm hoping for Judge Solomon next time.')

That next date with the authorities is imminent; he thinks it might be 2 August, the day after the new law that has been drafted specifically to remove him comes into force. The law, part of the Serious Organised Crime Bill, will, by the by, also remove 350 years of rights to peaceful protest within a quarter of a mile of the Palace of Westminster.

Not surprisingly, there are few subjects you can raise with Brian that do not lead quickly to 'Tony Bliar' and 'Looney Toon Hoon' and the 'United Assassins of America'. While he talks, I cling to bits of stray information like driftwood and try to lash them together. In this way, very slowly, over the course of much of a day, I begin to hear the story of Brian Haw's life.

He was born in 1949, a twin, by 25 minutes the eldest of five children. The family lived for a while in Barking, Essex, and then moved to Whitstable in Kent. They were involved in an evangelical church; Brian found his faith at Sunshine Corner on the shingle beach next to the Oyster Company.

His father had been a sniper in the Reconnaissance Corps during the war and was among the first to enter Bergen-Belsen after its liberation. He never spoke much about it, Brian says, except to tell his son that you don't get peace by shooting people.

'Afterwards, he worked in a betting office. He was sharp enough to know you could not beat the odds, but did he damn well try! Eventually, he dipped his hands in the till, brought shame on our family. And then he gassed himself, 20 years after he had seen Bergen-Belsen. I wonder why?'

Having been apprenticed to a boatbuilder at 16, Brian became the head of the family. He joined the Merchant Navy, sending home £4 a week. He worked as a deckhand and eventually received his certificate to steer 27,000-ton ships. He passed through the Suez Canal, climbed the Pyramids, toured the ports of the Middle East and India, began to get some of his singular political education.

He returned from one voyage to do six months at a college of evangelism in Nottingham, real fire and brimstone stuff. After that, he decided to put himself where troubles were.

'I was in Ireland at Christmas, 1970,' he says. 'I was a boy of 20, with my guitar, singing carols, just walking the streets in and out of the Falls Road and the Shankill; giving out white balloons in republican pubs.'

He laughs a bit. 'I would stand between the two sides when they threw rocks at each other, playing my guitar like an eejit.'

On his return, he moved back to Essex, set up a removals business, worked part time as a carpenter. He met his wife, Kay, there, who lived across the road. They had four children, but Brian never lost his sense of mission. In 1989, powerfully affected by the films of John Pilger, and, with just £100 in his pocket, he set off for the killing fields of Cambodia. He wanted to witness, you begin to guess, something like his father had witnessed at Bergen-Belsen, to find out how it would affect him, if it would explain anything.

I wonder what his wife made of his departure?

'I left her four flights up in a maisonette in Barking,' he says. 'She was pregnant with our fifth child; it was winter and we were under threat of eviction. But I went with her blessing. She knew how much it meant to me.'

He travelled via Berlin, walked through the Brandenburg Gate on the night the wall came down, bought a ticket for 26 quid to get from Budapest to Beijing on the Trans-Siberian Railway.

'I sat on a luggage rack showing the Chinese pictures of all my children; they thought I must be a millionaire.'

He was in Cambodia for three months, in search of horror; when he got home he wanted to tell the world what he had seen.

'My church gave me,' he recalls, bleakly, '10 minutes in a midweek prayer meeting to talk about genocide.' So he embarked on his personal kind of crusade. He and his ever-expanding family - he and Kay now had seven children - ended up on an estate in Redditch in Worcestershire, and it was there, it seems, that his problems escalated.

Brian had a minivan, he says, for his missionary work. He gave it a beautiful metallic paint job, took difficult local kids out in it along with his family, for days out. He was repaid, he suggests, by the families he sought to help with bricks through his window and fireworks through the letterbox while his little daughter was sitting doing her homework. Brian compiled a dossier on his problem neighbours for the CPS but it made matters worse; his minivan was smashed up beyond repair.

It was not long afterwards, following his faith, that he came here. There is, I suggest, an irony in this which he does not acknowledge: having been the victim of neighbours from hell, he has now been branded as one.

He has vowed to stay here as long as it takes. His wife, he believes, supported him for the first year, but has now been granted a divorce, despite the fact that he feels 'closer to her than I have ever been in my heart'. He has, he admits, nowhere now to go.

People ask him how he can abandon his children; he heard last week that his daughter was getting married. He says that he loves everyone's kids as much as his own. 'Is that not what all fathers should feel?'

I wonder if he believes, given the new law, that the end of his vigil is inevitably in sight. He says he thinks he has a strong human-rights case. 'I have been told by Chief Inspector Andy Robinson,' he says, 'who came down here dressed in his suit, paying me a courtesy call, that they won't do anything untoward.'

Aside from the police threat, he faces almost nightly informal eviction orders by drunks. He has had his nose broken three times, once by a man who introduced himself, he says, as being from Israeli military intelligence ('I told him that was a contradiction in terms').

Some people tell him he has made Parliament Square an eyesore. 'I tell them if they want to see an eyesore, then look at the picture from Fallujah, 90,000 homes destroyed, the pictures from Abu Ghraib'

While we speak, I get some sense of the way his presence divides people. The majority of passers-by stop to congratulate him. A distinguished looking American promises to supply him with a 'troops out' hoodie ('Might as well be hung for a sheep as a lamb'). A man in a passing Humvee shouts: 'Wanker.'

After the London bombs, a few people wondered, from passing cars, if he was satisfied now.

'Silly, silly people,' he says. 'Don't they realise I am against all bombs? Bombs in London and our bombs in Baghdad ...'

While Brian is still flying, a young community policeman wanders over and asks if he has seen anything untoward this morning.

'Brian's often our eyes and ears in Parliament Square,' he tells me. 'Not sure how we'll cope when he has gone.'

Brian smiles. 'You won't be getting rid of me that easily,' he says.

And he hunkers down again in his deckchair, looking out from under his beanie hat at the mother of parliaments.